What key shortcuts are to comment and uncomment code?

There were Ctrl+E+C (comment) and Ctrl+E+U (uncomment) in older versions, or Ctrl+K+C and Ctrl+K+U.

But in VS 2012, I can't see key shortcuts:

How to enable those shortcuts?

Keyboard accelerators are configurable. You can find out which keyboard accelerators are bound to a command in Tools -> Options on the Environment -> Keyboard page.

These commands are named Edit.CommentSelection and Edit.UncommentSelection.

(With my settings, these are bound to Ctrl+K, Ctrl+C and Ctrl+K, Ctrl+U. I would guess that these are the defaults, at least in the C++ defaults, but I don't know for sure. The best way to find out is to check your settings.)

You can also add the toolbar in Visual Studio to have the buttons available.

View > Toolbars > Text Editor

Use the keys CtrlK,C to comment out the line and CtrlK,U to uncomment the line.

From your screenshot it appears you have ReSharper installed.

Depending on the key binding options you chose when you installed it, some of your standard shortcuts may now be redirected to ReSharper commands. It's worth checking, for example Ctrl+E, C is used by R# for the code cleanup dialog.

This is how I did it,

Menu Tools → Options on the Environment → Keyboard window

One can alter the default shortcuts following the below steps

• Select Edit.CommentSelection in the listbox
• Click on "Remove" button
• Select "Text Editor" option in the dropdown under "Use new shortcut in:"
• Press your own shortcut in the textbox under "Press shortcut keys:" Example: Pressing Ctrl+E and then C will give you Ctrl+E, C
• Click on "Assign" button
• Repeat the same for Edit.UnCommentSelection (Ctrl+E, U)